1. Field of the Invention
The present invention relates to an apparatus and method to improve gas backwash, typically air backwash, in filters with combined gas/liquid (e.g., air/water) lateral type underdrain systems.
2. Prior Art
It is conventional in filters, such as gravity filters having a bed of filter media for treating water and wastewater, to provide an underdrain beneath the filter media for conveying filtered liquid out of the filter and for periodically distributing a cleansing liquid and/or gas to "backwash" the filter media, providing longer filter life. In air/water backwashing of filters with dual lateral air/water underdrain systems, such as disclosed in U.S. Pat. No. 4,065,391 to Farabaugh, it has been found desirable to have an air-only step in a sequence of air and water combinations. The air is applied beneath the filter media, and it vigorously agitates and breaks up dirt in the filter media in its ascent to the surface. The dirt is then more easily removed from the filter during water backwash. Net water usage is thus decreased. It has become quite common for gravity filter designers and users to specify an initial air-only backwash segment or phase when requesting vendors to supply new or retrofitted filter underdrain equipment, because it lowers overall operating and installation costs. Gases or liquids in place of or in addition to air or water may also be used for backwashing filters, but for simplicity, "air" and "water" will be used interchangeably with "gas" or "liquid" throughout this specification.
In the air-only backwash phase, the lateral underdrain and filter media are typically submerged in the process water. Air is pumped into the underdrain, and the pressurized air forms a pocket in the water below the upper deck or top wall of the underdrain. The air escapes through dispersion apertures in the upper deck and rises through the media to the water surface. On its way it rigorously separates the dirt from the media.
To maximize filter performance and operating longevity, it is important that the air/water lateral underdrain supporting the filter media meet the following design objectives:
a. provide close spacing of filtrate collection and backwash air/water distribution apertures (openings) in the top deck of the lateral underdrain system;
b. engage both backwash air and water to the maximum filter media area and depth to avoid pockets of unengaged media; and
c. provide distribution apertures which are sufficiently large to prevent clogging.
The referenced Farabaugh underdrain accomplishes these objectives by passing both backwash water and backwash air through the same apertures, which are primarily designed for water distribution, in the top deck of the underdrain. However, we have recently discovered that dual use of apertures in the Farabaugh underdrain has led to a phenomenon known as "drag water return" during air-only backwashing. Previously unknown to the inventors and others skilled in the art, this phenomenon has impeded the ability of the Farabaugh underdrain to achieve its maximum distribution potential during air-only backwash, especially at lower air flow rate. It has also limited the lower end of the range of backwash air flow rates under which the underdrain can properly distribute air. Experience has shown the Farabaugh underdrain requires a minimum air flow rate of 3 cubic feet per minute per square foot ("cfm/ft.sup.2 ") to achieve proper air backwash distribution.
Due to the dual use feature, the dispersion apertures in Farabaugh must be sized so that their total cross-sectional area is able to provide proper headloss for both backwash water and backwash air. Enlarging the cross-sectional area will result in poor air distribution, and downsizing or reducing the cross-sectional area will cause the pressure drop during water backwash to be too high. Particularly, any underdrain system must be designed on the basis of a certain minimum hydraulic headloss to establish optimum backwash water distribution. For fluidized media applications, such headloss must generally be greater than the clean filter media headloss at its incipient water fluidization point, where full media fluidization is expected (e.g., 10-30 gallons per minute per square foot ("GPM/ft.sup.2 ")). On the other hand, the backwash water headloss must not be so high as to require undue energy to achieve appropriate backwash water flow. Conventional lateral underdrain systems will provide optimum water backwash performance tip to a maximum headloss of water through the entire underdrain, including passage through the discharge apertures into the media, of 15-40 inches of water column at about 20 GPM/ft.sup.2 backwash flow rate. This is roughly the maximum conventional headloss under which most current lateral underdrains will efficiently perform on water backwash. Higher headloss results in a great waste of energy and increases the power requirements and pump sizing for a water treatment process. These general headloss guidelines at common liquid backwash flow rates establish the specific number and size of all discharge aperture openings in the top deck of the lateral underdrain. Preferably, the sizing and spacing for the apertures may be maintained the same for all applications of the underdrain to simplify design and manufacturing requirements.
When the apertures which distribute backwash air are the same as the optimum sized and spaced non-clog apertures which distribute backwash water, the headloss through such top deck apertures distributing air is significantly lower because of the much lower density of air. This lower headloss is enough to form an air pocket beneath the top deck, but it does not allow a sufficiently deep air pocket to form in front of the apertures so that air and water can effectively separate before entering the aperture.
We have recently discovered that the high velocity air by its jet action will drag some liquid through the pocket and up through the dispersion apertures to the media side of the lateral, creating a pressure void in the conduit or lateral chamber below the top deck. This pressure void causes an equivalent water amount to return continuously downward through the dispersion apertures to maintain pressure equilibrium on either side of the underdrain's top deck. Such "drag water return" will block some apertures in the top deck from distributing any air at all, and others will distribute air only intermittently. This phenomenon has been confirmed using dye tests in a transparent pilot filter, both with and without the features of the present invention.
Several additional U.S. patents disclose combined air/water lateral underdrains, including U.S. Pat. Nos. 5,160,614; 5,156,738; 5,149,427; 5,108,627; 5,087,362; 5,068,034; 5,019,259; 4,331,542; 4,214,992; 4,196,079; 4,064,050; 3,468,422; and 2,710,692. However, none of the prior art of which we are aware at this time addresses the problem of drag water return during air-only backwashing in underdrains which utilize the same apertures for both air and water distribution.
Thus, it is an object of the present invention to avoid air blockage in certain apertures in the top deck of an air/water underdrain lateral during air-only backwashing. It is a further object to provide a passage for unobstructed drag water return from above the underdrain to below the pressurized air blanket. It is a still further object to significantly improve the backwash air distribution when using the same apertures to disperse both backwash water and backwash air, without impeding the filtrate collection and backwash water distribution performance of the lateral underdrain. Finally, it is an object of the present invention to improve backwash air distribution at very low air-only rates, such as 1-2 cfm/ft.sup.2 over both short and long lateral lengths. Backwashing at lower air-only rates is sometimes desirable to enable the use of blower equipment with lower ratings, thus reducing the installation and operational costs associated with air backwashing systems.